Gallium-containing glassy low dielectric ceramic compositions

ABSTRACT

A ceramic composition for forming a ceramic dielectric body having a dielectric constant of less than about 5.5 and a TCE of less than about 4.5 ppm/° C. The composition comprises a mixture of finely divided particles of 25-50 vol. % borosilicate glass 40-75 vol. % silica glass and 1-40 wt. % of a material selected from the group consisting of Ga 2  O 3 , Ga 2  SiO 5 , Ga 2  TiO 5 , GaAs, GaPO 4  and any gallium-contained compounds to inhibit the formation of crystalline forms of silica. The composition can be used with a polymeric binder to produce an unfired green tape which is co-fireable with high conductivity metallurgies such as gold, silver and silver/palladium.

FIELD OF THE INVENTION

The invention relates to dielectric compositions. More particularly theinvention relates to glass and ceramic materials that are sintered atlow temperatures to produce dense bodies having low coefficients ofthermal expansion and a dielectric constant below 5.5.

BACKGROUND OF THE INVENTION

Conventionally, alumina (Al₂ O₃) is used as a dielectric material formicroelectronic packages. It has excellent electrical (insulating),thermal and mechanical (especially strength) properties. Alumina basedpackages, generally containing 4-10 wt. % glass, require sinteringtemperatures above 1500° C. which necessitates the use of refractorymetals such as molybdenum or tungsten for the electricalinterconnections so that the metal can be co-fired with the package.These metals have poor electrical conductivity as compared to highlyconductive metals such as copper, and secondly, they require the use ofstrongly reducing atmospheres during co-firing necessitating expensivefurnace systems.

The development of multilayer ceramic circuit boards is toward higherfrequency, higher density and higher speed devices. Al₂ O₃ has arelatively high dielectric constant of about 9.9, causing high signalpropagation delay and low signal-to-noise ratio (crosstalk). The signalpropagation delay (t) in ceramic substrates is affected by the effectivedielectric constant of the substrate (k') in the following equation:

    t=(k')0.5/C

where C is the speed of light. It can be found that the signalpropagation delay can be dramatically reduced by a reduction in theeffective dielectric constant of the substrate. For example, if thedielectric constant of a material is reduced from 10 (approximately thek' of Al₂ O₃) to 5, the signal propagation delay can be reduced by 30%.A small signal delay is especially important for the substrate housing achip with a very dense integrated circuit, for instance, very largescale integrated circuit (VLSI).

Furthermore, alumina has a coefficient of thermal expansion of about7.4×10⁻⁶ /° C. (in the 20°-200° C. range) as compared with 3.4×10⁻⁶ /°C. for silicon. This mismatch in thermal expansion results in designconstraints and reliability concerns when attaching a silicon wafer tothe substrate.

Heretofore, most of the dielectric materials used in multilayer circuitshave been conventional thick film compositions. A typical circuit isconstructed by sequentially printing, drying and firing functional thickfilm layers atop a ceramic substrate which is usually 92-96 wt. % Al₂O₃. The multiple steps required make this technology process intensivewith the large number of process steps and yield losses contributing tohigh costs. Thick film technology nevertheless fills an important needin microelectronics and will continue to do so in the foreseeablefuture.

Recently, dielectric thick film compositions with low dielectricconstants of 5 have been introduced. However, ceramic substrates withlow dielectric constants less than 5.5 and thermal expansioncoefficients which closely match that of silicon (3.4 ppm/° C.) are notreadily available.

Low temperature co-fired (LTCF) technology has been recently introducedas a method for fabricating multilayer circuits. This technology offersthe combination of the processing advantages of HTCF technology and thematerials advantages of thick film technology. These LTCF tape systemshave firing temperatures below 1000° C. and allow the use of highconductivity metals such as silver, gold, silver/palladium and copper(copper, however, requires reducing atmospheres). Most of these tapesystems have dielectric constants between 6 and 8 and encompass a rangeof thermal coefficient of expansion (TCE).

Currently, there is no readily available low temperature co-fireddielectric tape system using a glass plus ceramic approach that offersboth low dielectric constant (less than 5.5) and a TCE matched tosilicon (3.4 ppm/° C.).

PRIOR ART

A method for producing a multilayer ceramic circuit board for use withcopper conductors is described in U.S. Pat. No. 4,642,148, issued toKurihara et al. Ceramic compositions comprising 10-75 wt. %alpha-alumina, 5-70 wt. % non-crystalline quartz (fused silica), 20-60wt. % borosilicate glass are disclosed. The dielectric constants of thefired materials ranged from 4.8 to 9.6.

U.S. Pat. 4,672,152, issued to Shinohara et al, describes a multilayerceramic circuit board in which the ceramic is prepared from a mixture of50-95 wt. % crystallizable glass and 5-50 wt. % ceramic filler. Thematerial has a dielectric constant between 5.1 and 6.0 and a flexuralstrength above 150 MPa. The crystallizable glass consists of 5-20 wt. %lithium oxide, 60-90 wt. % silicon dioxide, 1-10 wt. % aluminum oxideand 1-5 wt. % alkaline metal oxide other than lithium oxide. The ceramicfiller is selected from the group of silicon dioxide, β-eucryptite(LiAlSiO₄) and aluminum oxide.

U.S. Pat. No. 3,926,648, issued to Miller, describes a process forsintering powdered crystallizable glasses having compositionsapproximating the stoichiometry of cordierite (2MgO·2Al₂ O₃ ·5SiO₂) intocordierite. The cordierite bodies exhibit low coefficients of thermalexpansion and contain hexagonal cordierite as the crystal phase.

U.S. Pat. No. 4,755,490, issued to DiLazzaro, describes a low firingtemperature ceramic materials having dielectric constants between 4.5and 6.1. The materials had coefficient of thermal expansion between 3.9and 4.2 cm/cm/° C.×10⁻⁶. Example 11 shows k'=4.5 and TCE=3.9. Thematerial is formed from a mixture of 10-50 wt. % alumina, 0-30 wt. %fused silica and 50-60 wt. % (approximately 60-70 vol. %) of a fritcomposed of about 4 wt. % CaO, about 12 wt. % MgO, about 29 wt. % B₂ O₃,and about 42 wt. % SiO₂ The compositions are fired at a temperaturebelow 1000° C.

U.S. Pat. No. 4,788,046, issued to Barringer et al, describes aglass-ceramic packages for integrated circuits having low sinteringtemperature. The sintered compositions are formed by coating ceramicparticles with glass, separating the coated particles from the glass andthen forming the coated particles into a green compact. The materialwith the lowest dielectric constant (4.5) is obtained using quartz. Thismaterial has had a thermal expansion coefficient greater than 5.5.

U.S. Pat. No. 4,849,379, issued to McCormick, describes a compositionfor making low dielectric layers which is an admixture of finely dividedsolids. McCormick states that materials such as cordierite and mulliteare not suitable for use on Al₂ O₃ substrates because of TCE mismatch.In addition, McCormick states that compositions containing cordieriteand mullite in conjunction with a low softening point glass in generaltend to raise TCE, lower firing temperature and increase the dielectricconstant of the composition.

U.S. Pat. No. 4,879,261, issued s a low dielectric material having adielectric less than 5.0. The material is formed from a mixture offinely divided particles consisting essentially of 70-85 wt. % silicaand 15-30 wt. % zinc borax flux which is fired to 1065° C. in anoxidizing atmosphere. The composition can be used to make green tape andmultilayer devices having internal copper conductors such as multilayercapacitors and multilayer interconnects.

From the foregoing, it can be seen that there is a substantial need fora low temperature co-fireable tape dielectric which (1) has a lowdielectric constant (less than 5.5), (2) has a thermal expansioncoefficient very close to the value for silicon (3.4 ppm/° C.), and (3)can be fired in air at a low temperature (less than 950° C.), thuspermitting the use of high conductivity metallurgies such as gold,silver and silver/palladium.

The principal object of the invention is to provide a material that canbe sintered into a body that has a dielectric constant of less than 5.5at 1 MHz, and a thermal expansion coefficient which closely matches thatof silicon.

Another object of the invention is to provide ceramic materials that aresintered at temperatures less than 950° C. for 4-20 hours withoutsignificantly increasing their thermal coefficient of expansion.

Another object of the invention is to provide ceramic materials that aresintered at low temperatures to produce dense bodies (greater than 95%of theoretical density) having low coefficients of thermal expansion anda dielectric constant below 4.5 and have a glass content below 50 vol.%. A reduction in the glass content of the sintered body is verydesirable in that the glassy phase is responsible for shape distortionor warpage during co-firing. If the sintered body is to be used in anelectronic package, the shape distortion associated with high volumepercent glass content can cause the via holes to misalign duringco-firing of the metal and ceramic. A glass content below 50 vol. % willreduce the likelihood that warpage will occur.

SUMMARY OF THE INVENTION

The invention is directed to a ceramic composition for forming a ceramicdielectric body having a dielectric constant of less than about 5.5 at 1MHz and a TCE which closely matches that of silicon, 3.4 ppm/° C., thecomposition being co-fireable with high conductivity metals such asgold, silver and silver/palladium. The composition comprises a mixtureof 20-50 vol. % borosilicate glass, 40-75 vol. % of a glass selectedfrom the group consisting of glass containing 95-98 vol. % silica,titanium silicate glass and combinations thereof and sufficient amountsof a gallium-containing material to inhibit the formation of crystallineforms of silica. In a preferred embodiment of the invention thegallium-containing material is incorporated into the mixture as a 1-40vol. % addition from the group consisting of Ga₂ O₃, Ga₂ SiO₅, Ga₂ TiO₅,GaAs and GaPO₄.

In a second aspect, the invention is directed to an unfired green tapecomprising the composition formed from the above identified mixturedispersed in a polymeric binder that can be fired for periods of timewell in excess of four hours without increasing its TCE.

In a further aspect, the invention is directed to a multilayer ceramicsubstrate comprising layers of the above composition and interconnectedconductor layers of copper therebetween, the assemblage having beenfired in excess of four hours to form a dense hermetic structure.

In a yet another aspect, the invention is directed to a multilayerceramic capacitor comprising layers of the above composition withconductor layers of copper therebetween, the assemblage having beenfired to form a dense hermetic structure.

BRIEF DESCRIPTION OF THE DRAWING

Other features of the present invention will be further described orrendered obvious in the following relating description of the preferredembodiments which is to be considered together with the accompanyingdrawing, wherein:

FIG. 1 is a graphical illustration of dimensional change versustemperature.

DETAILED DESCRIPTION OF THE INVENTION

The preferred glass plus ceramic composition of the present inventioncomprises a mixture of two principal components: borosilicate glass anda glass selected from the group consisting of titanium silicate glassand high silica glass. The percentages of each component may be variedwithin the ranges delineated below, depending on the final desiredproperties of the fired ceramic material. In addition to the twoprincipal components, the present invention includes sufficient amountsof a third material, gallium-containing material, which acts to suppressthe formation of crystalline forms of silica during the firing of themixture of the borosilicate and high silica glass.

Dense ceramic bodies can be formed from such compositions by normalmanufacturing techniques and low temperature (i.e., 850°-1000° C.)sintering. In a preferred application of the invention, such a mixtureis formed into a thin tape, via holes punched through the tape atdesired locations, and one or more metal conductor paths are formed onthe punched tape. Suitable metals for the conductor paths includecopper, silver, gold, platinum/gold and palladium/silver. The tape issubsequently sintered at low temperature, typically after two or moresections have been laminated together to form a multilayer circuitsubstrate.

It has been found that low firing temperature glass plus ceramiccompositions can be made from mixtures containing less than 50 wt. %borosilicate glass. As stated above, a reduction in the glass content ofthe sintered body is very desirable in which the glassy phase isresponsible for shape distortion or warpage during co-firing. A glasscontent below 50 wt. % will reduce the likelihood that warpage andmisalignment of via holes will occur. Low firing temperature glass plusceramic compositions of the invention are produced by providing amixture of powdered ingredients, including 25-50 wt. % borosilicateglass and 50-75 wt. % high silica glass and sufficient amounts ofcrystalline ceramic materials to inhibit the formation of crystallineforms of silica.

The borosilicate glass is composed of Al₂ O₃, B₂ O₃ and SiO₂ in amountssuch that the mixture has a softening point of about 810° C. A quantityof the mixture is then formed into a desired shape using conventionalprocedures, and sintered at a temperature of at least 850° C.,preferably 850°-950° C., and most preferably 900°-950° C. The sinteringmay be conducted in an oxidizing, neutral or reducing atmosphere.

The term "glass plus ceramic" is used herein to describe a sinteredceramic composition which is formed from a mixture of crystallineceramics and glass. The ceramic and glass phases of the glass plusceramic composition remain distinct after firing. The glass in a glassplus ceramic system retains its glassy characteristic after firing andis said to be a non-crystallizable glass in that composition. Theceramic in a glass plus ceramic system need not be a crystallinematerial; it may also be a glass. The ceramic, whether glassy orcrystalline in nature, retains its initial characteristic after firingand is said to behave as a ceramic in that fired composition. Inaddition, the term "glass plus ceramic" is used herein to distinguishsystems containing non-crystallizable glasses from "glass-ceramic"systems in which the glass undergoes a controlled devitrification duringfiring and becomes crystalline.

The term "borosilicate glass" is used herein to describe a family ofglasses containing 10-20 wt. % boron oxide (B₂ O₃) and 75-85 wt. %silicon oxide (SiO₂) Other oxides commonly found in borosilicate glassinclude Al₂ O₃, CaO, K₂ O, Li₂ O and Na₂ O in amounts such that themixture has a softening point of about 800° C.

The term "high silica glass" is used herein to describe a family ofglasses containing greater than 95 wt. % silicon oxide (SiO₂) andcontains 3-4 wt. % B₂ O₃ and 0-1 wt. % Al₂ O₃. "High silica glass" has asoftening point greater than 1500° C. and does not devitrify when usedin a ceramic composition which is fired below 1000° C. "High silicaglass" can, therefore, be said to behave like a crystalline ceramicfiller since it remains distinct from the other ceramic components ofthe material. When the term "glass plus ceramic" is used in reference tothe present application, the "high silica glass" is the ceramiccomponents.

The term "titanium silicate glass" is used herein to describe a familyof silicates containing 1-20 wt. % TiO₂ and 80-99 wt. % SiO₂. Both thehigh silica glass and the titanium silicate glass have a softening pointgreater than 1500° C. and do not devitrify when used in a ceramiccomposition which is fired below 1000° C. They can, therefore, be saidto behave like a crystalline filler since they remain distinct from theother ceramic components of the material.

The cristobalite and quartz phases formed during firing remains in thematerial on cooling. Cristobalite has a TCE of about 50×10⁻⁶ /° C. (inthe 20°-300° C. range) and quartz has a TCE of about 13×10⁻⁶ /° C. ascompared to 3.5×10⁻⁶ /° C. for silicon. The presence of cristobaliteand/or quartz in the fired product raises the TCE and lowers themechanical strength of the product. The loss of mechanical strength isdue to the volume change associated with phase transformation whichgenerates microcracks.

The term "crystalline ceramic material" is used herein to describe afamily of refractory ceramic materials containing low levels of elementsselected from group IA of the periodic table. The term "crystallineceramic material" is intended to include, but is not limited to Ga₂ O₃,Ga₂ SiO₅, Ga₂ TiO₅,GaAs, GaPO₄ and any gallium-contained compounds. Theterm "crystalline ceramic material" is not intended to include thevarious crystalline forms of silica (SiO₂) which include quartz,tridymite, flint and cristobalite. As stated above, the presence ofcrystalline phases of silica, such as quartz and cristobalite, remain inthe material during firing and on cooling and its presence in the firedproduct raises the TCE and lowers the mechanical strength of theproduct. Linear thermal expansion coefficients for polymorphoric formsof silica and glasses are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                               Thermal Coefficient of Expansion                                       Composition                                                                            20-100° C.                                                                       20-200° C.                                                                       20-300° C.                                                                     20-600° C.                        ______________________________________                                        Quartz   11.2      --        13.2    23.7                                     Cristobalite                                                                           12.5      --        50.0    27.1                                     Tridymite                                                                              17.5      --        25.0    14.4                                     Fused Silica                                                                           --        0.5       --      --                                       Glass                                                                         High Silica                                                                            --        0.7       --      --                                       Glass                                                                         Borosilicate                                                                           --        3.3       --      --                                       Glass                                                                         ______________________________________                                    

The term "finely divided" is used herein to describe material that isground to less than about 5 microns in size.

The glasses can be prepared by conventional glass-making techniques bymixing the desired components in the desired proportions and heating themixture to form melt. As is well known in the art, heating is conductedto a peak temperature and for a time such that the melt becomes entirelyliquid and homogeneous.

The above-described glasses are particularly desirable for use inelectronic packages, such as VLSI applications, because of their lowpolarizability and thus low dielectric constant. Because borosilicateglasses by themselves tend to have low softening points, it is necessaryto increase the softening point by the addition of large amounts ofother glasses which have high SiO₂ concentrations. High silica glassesare more durable than those with high B₂ O₃ concentrations.

A preferred borosilicate glass is sold under the tradename "Pyrex GlassBrand No. 7740" and is commerically available from Corning Glass Works.This glass comprises about 0-3 wt. % Al₂ O₃, 10-20 wt. % B₂ O₃, 0-1 wt.% CaO, 0-1 wt. % K₂ O, 0-1 wt. % Li₂ O, 0-4 wt. % Na₂ O and 75-85 wt. %SiO₂. The amount of borosilicate glass used affects the sinteringtemperature. If too little borosilicate glass is used (for example, lessthan about 25 vol. % in this embodiment), the sintering temperature willbe too high to achieve the benefits of the present invention.Maintaining the proportion of borosilicate glass within the range ofabout 20-50 vol. % is necessary to obtain these benefits.

A preferred high silica glass composition is sold under the tradenameCorning 7913 and contains 0.5 wt. % alumina, 3 wt. % B₂ O₃ and 96.5 wt.% SiO₂.

The term "titanium silicate glass" is used herein to describe a familyof glasses containing 80-99 wt. % silicon oxide (SiO₂) and 1-20 wt. %TiO₂. Titanium silicate glass has a softening point of about 1400°-1500°C. depending on its composition. Since the titanium silicate glass doesnot soften when fired to temperatures below about 1000° C., it cantherefore be said to behave like a crystalline filler. Thus, the use oftitanium silicate glass will not contribute to shape distortion orwarpage during co-firing. As stated above, the shape distortionassociated with high volume percent glass content can cause the viaholes in the electronic package to misalign during co-firing of themetal and ceramic.

In addition to titanium silicate glass being refractory, it does notnormally devitrify when used in a ceramic composition which is firedbelow 1000° C. In this regard, titanium silicate glass, which usuallycontains about 93 wt. % SiO₂, is different from "fused silica glass"which is virtually 100% SiO₂.

The cristobalite and quartz phases formed during firing remain oncooling. Cristobalite has a TCE of about 50×10⁻⁶ /° C. (in the 20°-300°C. range) and quartz has a TCE of about 13×10⁻⁶ /° C. as compared to3.5×10⁻⁶ /° C. for silicon. The presence of cristobalite and/or quartzin the fired product raises the TCE and lowers the mechanical strengthof the product. The loss of mechanical strength is due to the volumechange associated with phase transformation which generates microcracks.Titanium silicate glass will not normally form cristobalite crystalliteswhen it is fired to temperatures below about 1000° C.

The following examples illustrate preferred ranges of components of theglass plus ceramic compositions of the invention. In each example, theborosilicate glass is comprised of 3.0 wt. % Al₂ O₃, 13.0 wt. % B₂ O₃,4.0 wt. % Na₂ O and 80 wt. % SiO₂ and the high silica glass is Corning's7913.

EXAMPLE 1

In this example, the starting materials consisted essentially of 50 vol.% high silica glass and 50 vol. % borosilicate glass. The borosilicateglass and the high silica glass were separately ground in a 1.3 gallonball mill for 16 hours to achieve a particle size of 2-4 microns. Sincethe density of the borosilicate glass and the high silica glass areapproximately the same, the volume percent is roughly equivalent to thewt. %. In this example the actual wt. % of the mixture is 49.4 wt. %high silica glass and 50.6 wt. % borosilicate glass. This mixture ofinorganic material was combined with 5 wt. % polyethylene glycol binderand 50 wt. % 1-propanol and mixed for 2 hours in a tubular mixer. Thematerial was then oven dried at 80° C. for 2 hours and screened. Thematerial was then dry pressed into 1.9 cm diameter, 0.3 cm high pelletsby compressing the milled mixture in a mold at 13,000 psi (910 kg/cm²).The pellets were then fired in air. The firing was in two steps. Thefirst step was to burn the binder out. This was accomplished by heatingthe pellets to 500° C. and holding for 1 hour. Next the pellets weresintered isothermally at 925° C. for 4 hours. Sintered density of thesintered materials was determined by the water replacement method,according to ASTM procedure C373-72. Dielectric constant and dissipationfactor were measured by an HP 4192 AC impedance at 1 MHz. Thermalexpansion coefficients (TCE) were determined in the temperature rangefrom room temperature to 200° C. using a dilatometer. Those results wererecorded in Table 2. As noted, a large TCE of 19.7 ppm/° C. and a strongpeak intensity of cristobalite (100) were observed.

                  TABLE 2                                                         ______________________________________                                                           Sin-                                                                          tered                                                           Compositions  Den-          TCE                                          Ex.  (vol. %)      sity    Crist.                                                                              (ppm/      DF                                No.  BSC    HSG    Ga.sub.2 O.sub.3                                                                    (%)   (cps) °C.)                                                                         k'   (%)                           ______________________________________                                        1    50     50      0    102   11468 19.71 --   --                            2    50     40     10    96.5   125  3.43  4.78 0.8                           3    50     30     20    95.8   130  4.41  5.28 0.8                           ______________________________________                                    

EXAMPLE 2

The procedure of Example 1 is repeated except that the inorganiccomposition was 40 vol. % high silica glass, 50 vol. % borosilicateglass and 10 vol. % Ga₂ O₃ (34 wt. %, 43 wt. % and 23 wt. %,respectively). The results of the sintered density, XRD analysis,thermal expansion coefficient and dielectric constant are listed inTable 2. As noted, this composition has a relative sintered density of96.5%, an observed intensity of cristobalite (100) peak of 125 cps, adielectric constant of 4.8 at 1 MHz and a TCE of 3.43 ppm/° C. which isvery close to that of silicon (see FIG. 1).

EXAMPLE 3

The procedure of Example 1 is repeated except that the inorganiccomposition was 30 vol. % high silica glass, 50 vol. % borosilicateglass and 20 vol. % Ga₂ O₃ (22 wt. %, 38 wt. % and 37 wt. %,respectively)and the firing temperature was 925° C. for 4 hours. Theresults of the sintered density, XRD analysis, thermal expansioncoefficient and dielectric constant are listed in Table 1. It has beenfound that this composition has a relative sintered density of 95.8%, anobserved intensity of cristobalite (100) peak of 130 cps, a dielectricconstant of 5.28 at 1 MHz and a TCE of 4.41 ppm/° C.

The products of Examples 2 and 3 illustrate that the growth ofcristobalite precipitate during firing can be significantly reduced bythe addition of a small amount of crystalline Ga₂ O₃. This result isfurther demonstrated in that the TCE decreases dramatically from the Ga₂O₃ -free sample, 19.7 ppm/° C. to the Ga₂ O₃ -added samples, 3.4-4.4ppm/° C. (see FIG. 1). Moreover, the TCE of the product of Example 2 isvery close to that of silicon (3.4 ppm/° C.), which is very desirablefor multilayer ceramic packaging.

The products of Examples 2 and 3 contain a low glass content (25-50 vol.%) which is much less than those reported in the literature (greaterthan 60 vol. %). A low glass content is very desirable, because theshape distortion can be avoided during co-firing. The products ofExamples 2 and 3 have high sintered densities (greater than 95% of thetheoretical density) obtained at temperatures of 800°-950° C. in air.The sintering temperatures are compatible with those of precious metals,e.g., Au and Ag-Pd, which will enable the compositions to be utilized ina co-firable ceramic/metal electronic packaging system.

In addition, the materials of Examples 2 and 3 have low dielectricconstants (4.8-5.3 at 1 MHz) and low dielectric losses (0.7-0.8% at 1MHz) which are very desirable to reduce the signal propagation delay inthe ceramic substrate.

The materials of Examples 2 and 3 can be used to form multilayer highfrequency circuit packages. To form dielectric layers for multilayerhigh frequency circuit packages, the starting materials are ground in aball mill until they have an average particle size of 2-4 microns. Aslurry is then formed by combining the finely ground powder with asuitable solvent and other conventional additives, such as a plasticizerand a binder, in a manner known in the art. The slurry is cast into thin"green" (unfired) sheets having a thickness of about 75 to 400 micronsusing a conventional doctor blading process, after which the greensheets are blanked into individual 125 mm square sheets or tapes. Viaholes next are formed in the green sheets by a die punching process. Theholes suitably may have a diameter of about 125 microns. A conductorpaste is applied in a desired pattern to the punched sheets using ascreen printing process. The paste is also applied within the via holesto form connections between conductor patterns. The principal metallicconstituent of the paste may be gold, silver, copper, silver/palladiumalloy, gold/platinum alloy, or other suitable materials. The printedgreen sheets are then stacked in a desired sequence using alignmentholes to insure correct positioning, and laminated together at 50°-100°C. under a pressure between about 35 and 250 kg/cm². Finally, thelaminated green sheets are fired at a temperature not exceeding 1000° C.to form dense, sintered ceramic multilayer circuit substrates. Thefiring may be done in air if the conductor metal is not susceptible tooxidation at the firing temperature. Such is the case, for example, withthe metals named above, except for copper, which requires a reducing orneutral atmosphere. Sheets formed in the manner described will have alower glass content (25-50 vol. %) and therefore a lower tendency to bowor warp.

The compositions of the present invention also can be used to formrigid, nonporous ceramic bodies by substantially conventionaltechniques. For example, the batch ingredients of any of the previousexamples are combined with water and organic binders, and ball milledfor a period of about 20 hours. The resulting slurry is spray dried toprovide a powder of substantially spherical particles. This powder canbe used to form bodies of various desired shapes by standard formingtechniques, such as dry or isostatic pressing. The bodies are then firedat a suitable temperature not exceeding 1000° C. to provide dense,sintered ceramic objects.

Although the invention has been described in terms of a high silicaglass, it is contemplated that other forms of silica may be used inpracticing the present invention. However, it is not believed thatquartz and/or cristobalite can be used because of their high TCE.

Although the invention has been described in terms of using acrystalline Ga₂ O₃ as a crystal growth inhibitor, other forms ofcrystalline materials containing low levels of alkali ions may also beused in practicing the present invention.

Although applicants do not wish to be bound by any theories, it ispresently believed that the mechanism of crystallization inhibition isrelated to the migration of alkali ions in the borosilicate glass to theinterface with the crystallization inhibitor. Photomicrographs of amicroprobe have revealed that when Ga₂ O₃ is used as a crystallizationinhibitor, sodium ion in the borosilicate glass migrates toward the Ga₂O₃ /glass interface during firing of the mixture. At the same time Ga⁺³from Ga₂ O₃ dissolves into the glass.

It is believed that the segregation of alkali ions in the glass towardGa₂ O₃ /glass interface suppresses the tendency of the glass to undergophase separation at or near the firing temperature mixture. This phaseseparation is believed to be a precursor to crystallization of theglass.

It is further believed that crystalline materials containing alkali ionswill reduce the migration of the alkali ions in the borosilicate glassand thus reduce the inhibition of crystal growth that would otherwise beexpected. Alkali ions such as sodium are known to increase thedielectric loss of ceramic, which is very undesirable. It is believedthat materials that are for all practical purposes alkali-free Ga₂ O₃,Ga₂ SiO₅, Ga₂ TiO₅, GaAs, GaPO₄ and any gallium-contained compounds maybe used as crystal growth inhibitors in practicing the presentinvention.

In addition, although the invention has been described in terms of usinga 1-40 vol. % of a grain growth inhibitor, other amounts may also beused in practicing the present invention. The key is that enoughcrystalline material be used to cause the desired inhibition of crystalgrain growth without introducing other undesirable properties.

It will be apparent to those skilled in the relevant art that variouschanges and modifications may be made in the embodiments described aboveto achieve the same or equivalent results without departing from theprinciples of the present invention as described and claimed herein. Allsuch changes and modifications are intended to be covered by thefollowing claims.

What is claimed is:
 1. A ceramic composition comprising a mixture ofparticles of:(a) 25-50 vol. % borosilicate glass; (b) 50-75 vol. % highsilica glass; and (c) 1-40 vol. % of a gallium containing material. 2.The ceramic composition of claim 1 in which said gallium containingmaterial is selected from the group of Ga₂ O₃, Ga₂ SiO₅, Ga₂ TiO₅, GaAs,GaPO₄ and mixtures thereof.
 3. The ceramic composition of claim 1 inwhich said gallium containing material is 1-20 wt% Ga₂ O₃.
 4. Theceramic composition of claim 1 in which the borosilicate glass comprises0-3 wt. % alumina, 10-20 wt. % B₂ O₃, 0-3 wt. % CaO, 0-3 wt. % K₂ O, 0-3wt. % Li₂ O, 0-4 wt. % Na₂ O and 75-85 wt. % SiO₂.
 5. The ceramiccomposition of claim 1 in which the high silica glass comprises 0-1 wt.% alumina, 0-5 wt. %, B₂ O₃, 95-98 wt. % SiO₂, the remainder incidentalimpurities.
 6. A castable ceramic composition comprising:(a) 30-84 wt. %of a mixture comprising:25-50 vol. % borosilicate glass and 50-75 vol. %high silica glass; (b) 15-30 wt. % of an organic medium comprised of apolymeric binder dissolved in an organic solvent; and (c) 1-40 wt. % ofa gallium containing material.
 7. The ceramic composition of claim 6 inwhich said gallium containing material is selected from the group of Ga₂O₃, Ga₂ SiO₅, Ga₂ TiO₅, GaAs, GaPO₄ and mixtures thereof.
 8. The ceramiccomposition of claim 6 in which said gallium containing material is 1-20wt% Ga₂ O₃.
 9. A ceramic composition for forming a ceramic dielectricbody having a dielectric constant of less than about 4.2, saidcomposition comprising borosilicate glass, silica glass, and 1-40 vol. %of a gallium containing material.
 10. The ceramic composition of claim 9in which said gallium containing material consists essentially ofmaterials selected from the group of Ga₂ O₃, Ga₂ SiO₅, Ga₂ TiO₅, GaAs,GaPO₄ and mixtures thereof.
 11. The ceramic composition of claim 9 inwhich said gallium containing material is 1-30 vol. % Ga₂ O₃.
 12. Amethod of making a ceramic dielectric body having a dielectric constantof less than about 5.5, comprising the steps of:(a) providing a mixtureconsisting essentially of finely divided particles comprising 25-50 vol.% borosilicate glass, 50-75 vol. % high silica glass and 1-40 vol. % ofa gallium containing material; and (b) sintering the mixture in air to atemperature not greater than about 1000° C.
 13. The ceramic compositionof claim 12 in which said gallium containing material is selected fromthe group of Ga₂ O₃, Ga₂ SiO₅, Ga₂ TiO₅, GaAs, GaPO₄ and mixturesthereof.
 14. The ceramic composition of claim 12 in which said galliumcontaining material is 1-30 wt% Ga₂ O₃.